Fabrication of Porous Functional Nanonetwork-Structured Polymers

Article pubs.acs.org/cm

Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Fabrication of Porous Functional Nanonetwork-Structured Polymers with Enhanced Adsorption Performance from Well-Defined Molecular Brush Building Blocks Guojun Xie,†,§ Xidong Lin,‡,§ Michael R. Martinez,† Zelin Wang,‡ He Lou,‡ Ruowen Fu,‡ Dingcai Wu,*,‡ and Krzysztof Matyjaszewski*,† †

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Materials Science Institute, PCFM Lab and GDHPRC Lab, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P. R. China

Chem. Mater. Downloaded from pubs.acs.org by UNIV OF NORTH DAKOTA on 11/15/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: We report the fabrication of porous functional nanonetwork-structured polymers (FNSPs) consisting of poly(acrylic acid) functional cores surrounded by a microporous shell network, via a combination of atom transfer radical polymerization (ATRP) and Friedel−Crafts hyper-cross-linking chemistry. Well-defined molecular brushes with diblock poly(tert-butyl acrylate)-block-polystyrene (PtBA-b-PS) side chains serve as tunable building blocks to synthesize porous networks. The prepared FNSPs demonstrated significantly enhanced adsorption toward basic dyes, up to 767 mg g−1.

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INTRODUCTION Porous materials, including metal−organic hybrid networks,1 covalently cross-linked organic networks,2,3 zeolites,4 amorphous polymers,2,5 and porous carbons,6−8 have been a longterm subject of scientific interest due to their broad utility in energy, catalysis, adsorption, and biomedical applications. Porous polymers in particular have received growing attention due to their ability to combine the positive aspects of polymer synthesis and processing with the physical properties of nonpolymeric porous materials.9 One of the key advantages of porous polymers as compared to other porous materials is simple processability and feasibility of incorporating accessible functionalities while displaying high surface areas of welldefined size.5,9 Nanonetwork-structured materials (NSMs) are a new class of porous cross-linked polymeric materials that display macro-, meso-, and/or micropores within the same network.5,8 This hierarchical porosity results in greater pore accessibility and faster mass transport than exclusively macro-, meso-, or microporous materials. Previous studies have employed a standard sol−gel method to prepare NSMs with quasi-nonporous nanospheres as network units.10−13 Recent reports have investigated hyper-cross-linking of styrenic polymer building blocks in order to form complex nanonetwork-structured polymers and carbons with network units of solid microporous nanospheres,14−17 hollow microporous nanospheres,8,11 and microporous nanowires.5 However, the procedures previously used for preparation of most NSMs generally concentrated on the nanostructures of the network units without functionalities, which may restrict the utility of the NSMs for broader applications. © XXXX American Chemical Society

Molecular brushes are a class of polymer architecture with densely grafted side chains. The steric hindrance between the side chains induce the entire structure to adapt cylindrical conformations.18,19 The unique architecture of this type of polymer enables a broad range of applications, including anisotropic nanomaterials,20−24 lubricants,25−27 emulsifiers,28 drug carriers,29,30 and soft elastomers.31−35 Molecular brushes are strong candidates for precursors of functional nanonetworks due to their anisotropic structure and the flexibility of synthetic routes. Three synthetic strategies are available for the preparation of molecular brushes: “grafting-onto” (attaching preformed side chains to the backbone),36,37 “graftingthrough” (polymerizing macromonomers),38−40 and “grafting-from” (growing side chains from the backbone).41−44 The “grafting-from” method allows for a gradual growth of side chains from a functionalized backbone, resulting in brushes with significantly higher grafting density than the “graftingonto” method, and longer backbones than the “graftingthrough” method.34 Moreover, consecutive polymerization of two more monomers can create radially layered molecular bottlebrushes such as core−shell or double-shell cylindrical brushes.45−52 The composition of the building blocks is the key to the performance and applications of NSMs, which are defined by both the functionality and skeleton construction. Thus, we envision that well-defined molecular brushes could be utilized as building blocks for well-defined, compositionally Received: September 10, 2018 Revised: October 30, 2018

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DOI: 10.1021/acs.chemmater.8b03845 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Schematic illustration of the fabrication of porous functional nanonetwork-structured polymers (FNSPs) consisting of poly(acrylic acid) functional cores surrounded by a microporous shell network, via a combination of ATRP and Friedel−Crafts hyper-cross-linking chemistry. Welldefined molecular brushes with diblock poly(tert-butyl acrylate)-block-polystyrene side chains, i.e., P[BiBEM-g-(PtBA-b-PS)], serve as tunable building blocks to synthesize porous networks.

Table 1. Structural Parameters of Nanonetworks and Their Molecular Brush Building Blocks building block sample IDa

compositionb

f PAAc

Mn,thd

Mn,GPCe

Đe

NSP FNSP-PSS FNSP-PSL

P(BiBEM-g-PS220)1860 P[BiBEM-g-(PtBA42-b-PS52)]1860 P[BiBEM-g-(PtBA50-b-PS172)]1640

0 36 17

43 100 000 20 600 000 40 100 000

1 350 000 1 680 000 2 640 000

1.33 1.33 1.28

a NSP = nanonetwork-structured polymers from brushes with PS side chains; FNSP-PSS = functional nanonetwork-structured polymers from brushes with PtBA-b-PS side chains with short PS blocks; FNSP-PSL = functional nanonetwork-structured polymers from brushes with PtBA-b-PS side chains with long PS blocks. bDegrees of polymerization (DPs) were calculated based on the monomer/initiator ratio and conversion determined by 1H NMR in each step. cWeight fraction of PAA domain after being fully hydrolyzed. dCalculated based on composition. e Determined by GPC using linear PS.

3D interconnected hierarchical porous structure, including the micropores formed by intrabrush hyper-cross-linking of PS blocks and meso-/macroporous networks induced by interbrush hyper-cross-linking; and (iii) the incorporation of PAA linings, which were formed by in situ hydrolysis of the PtBA blocks during the hyper-cross-linking process, facilitates highly accessible positively charged dye binding sites, thereby affording enhanced adsorption capacity. This strategy could therefore provide new avenues for the customization of NSMs’ function and structure.

controlled, and functional NSMs. We previously reported the preparation of porous polymeric networks by cross-linking cylindrical polystyrene molecular brushes via a Friedel−Crafts reaction.5 The combination of intra/interbrush cross-linking generated networks exhibiting high specific adsorption capacities per unit surface area which were formed in response to the synergistic effect of the unique hierarchical porous structures. However, the networks showed limited functionalities due to the simple (polystyrene-based) chemical composition. Herein, this brush-based synthetic strategy is employed for preparation of functional porous materials by introducing functionality via incorporation of a poly(acrylic acid) (PAA) core to the copolymer grafts of brush building blocks. As shown in Figure. 1, the well-defined poly[2-(2-bromoisobutyryloxy)-ethyl methacrylate-graf t-(tert-butyl acrylate)-block-styrene] (P[BiBEM-g-(PtBA-b-PS)]) molecular bottlebrushes were prepared via atom transfer radical polymerization (ATRP).53−56 Functional nanonetwork-structured polymers (FNSPs) with PAA functionalities were then prepared via the Friedel−Crafts hyper-cross-linking process, leading to the formation of a nanonetwork with core−shell structured nanowire network units. The PtBA blocks were transformed into PAA functional groups via in situ hydrolysis. This strategy demonstrates several significant advantages: (i) highly controllable and predictable building blocks obtained via precise molecular design allow for preparation of a tunable network of functional units and porous structure; (ii) the FNSPs have a

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RESULTS AND DISCUSSION ATRP enables the precise design of P[BiBEM-g-(PtBA-b-PS)] molecular bottlebrushes with tunable molecular weights (Mn) and low dispersities for each building block of a nanoporous network. The diameter of each nanowire network unit is dictated by side chain length, which is tunable by the target degree of polymerization (DP) of each block grown from a linear multifunctional macroinitiator. The first synthetic step requires the synthesis of a PBiBEM macroinitiator via ATRP of 2-(trimethylsilyloxy)ethyl methacrylate (HEMA-TMS), followed by deprotection and modification of the precursor of the brush backbone with 2-bromoisobutyryl bromide (Figure S1 and Tables S1−S4 of the Supporting Information, SI). The sequential polymerizations of tert-butyl acrylate and styrene yielded core−shell structured molecular bottlebrushes with PtBA-b-PS diblock side chains. The structural parameters of the bottlebrush precursors are summarized in Table 1. For B


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Figure 2. (a) SEM and (b) TEM images of FNSP-PSL.

Figure 3. (a) N2 adsorption−desorption isotherm and (b) DFT pore size distribution curve of FNSP-PSL.

chain demonstrated a 3D continuous nanonetwork morphology composed of well-defined nanowire network units with a rough surface. The average diameter of the homogeneous nanowire network unit was 16 nm. Thermogravimetric analysis (TGA) measurements were performed to evaluate the alteration of thermal stability after hyper-cross-linking. As shown in the TGA curve in Figure S6, P[BiBEM-g-(PtBA50-bPS172)]1640 thermally fully decomposed before 500 °C, owing to its linear polymer chain structure. The introduction of −CH2− cross-linking bridges in FNSP-PSL resulted in a high weight yield of 51% at 600 °C, demonstrating a highly crosslinked and thermally stable skeleton. In addition, the hydrolysis of PtBA into PAA after hyper-cross-linking was verified by the appearance of a carboxylic acid signal at 3500 cm−1, the disappearance of the tert-butyl signal at 1368 cm−1 (sharp) and 1391 cm−1 (weak), and the shifting in carbonyl stretching from 1728 to 1719 cm−1 in the Fourier transform infrared (FTIR) spectra (Figure S7). The porosity of FNSP-PS L was quantified by N 2 adsorption−desorption isotherm (Figure 3a). The sharp uptake increases at low relative pressure indicated the presence of numerous micropores formed by the hyper-cross-linking of the PS shells, while the further increase at high relative pressure without reaching a plateau showed the existence of meso-/macropores, which could be ascribed to the interspace between cross-linked nanowire network units. The Brunauer− Emmett−Teller surface area (SBET) was measured to be 115 m2 g−1, and the micropore surface area (Smic) and meso-/ macropore surface area (Sext) were 63 and 52 m2 g−1, respectively. According to the pore size distribution curve

example, the DPs of the PBiBEM backbone, functional PtBA block and cross-linkable PS block in P[BiBEM-g-(PtBA50-bPS172)]1640 are 1640, 50, and 172, respectively (Figure S2). The gel permeation chromatography (GPC) trace in Figure S3 indicates well-controlled ATRP process, since the P[BiBEM-g(PtBA 50-b-PS 172 )]1640 trace exhibited a monomodal M n distribution with a low dispersity of 1.28. The scanning electron microscopy (SEM) image (Figure S4) shows the welldefined nanomorphology of P[BiBEM-g-(PtBA50-b-PS172)]1640. To fabricate FNSPs, the P[BiBEM-g-(PtBA-b-PS)] building blocks were hyper-cross-linked through a Friedel−Crafts reaction between PS block and formaldehyde dimethyl acetal (FDA) in the presence of anhydrous ferric chloride (FeCl3), which acted as a strong Lewis acid catalyst (Figure S5). Notably, the Lewis acid (FeCl3) not only worked as a catalyst for PS block hyper-cross-linking, but also reacted with PtBA block to hydrolyze and produce acrylic acid functionality in situ. In this process, the PS blocks underwent intra/interbrush Friedel−Crafts cross-linking through the formation of −CH2− cross-linking bridges between phenyl rings. It should be noted that intra/interbrush cross-linking played different roles in material design. Intrabrush cross-linking increased the modulus of the soft polymer, keeping the numerous as-formed internal micropores from collapsing after the removal of solvent. Meanwhile, interbrush cross-linking interconnected multiple brushes in various directions to fabricate nanonetwork structures with abundant interstitial meso-/macropores. As shown in the SEM and transmission electron microscope (TEM) images in Figure 2, the as-prepared FNSP-PSL from the P[BiBEM-g-(PtBA50-b-PS172)]1640 precursor with long side C


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Figure 4. (a) Adsorption curves and (b) percentage removal efficiencies toward methyl violet for FNSP-PSL (red), FNSP-PSS (blue), and NSP (gray) within 24 h. The adsorption capacity was calculated according to the equation C = (c0V0 − c1V1)/m, where c0, V0, c1, V1, and m represent the initial concentration, initial volume, concentration and volume after adsorption, and weight of adsorbents, respectively.

(Figure 3b) derived by density functional theory (DFT),57 the micropore size was about 1.3 nm and the meso-/macropores ranged from 2 to 170 nm, indicating the existence of a hierarchical porous structure. ATRP allowed for the precise design of P[BiBEM-g-(PtBAb-PS)] molecular bottlebrushes with different compositions, i.e., P[BiBEM-g-(PtBA42-b-PS52)]1860 and P[BiBEM-g-(PtBA50b-PS172)]1640 (Table 1). Both brushes were composed of long backbones, similar lengths of PtBA blocks, and different lengths of PS blocks in the side chains. Increasing the DP of PS blocks resulted in an increase of PS content from 64% in FNSP-PSS, obtained from P[BiBEM-g-(PtBA42-b-PS52)]1860, to 83% in FNSP-PSL. After the hyper-cross-linking reaction, the average diameter of nanowire network units of FNSPs increased from 10 to 16 nm via analysis of TEM images (Figures S8 and 2b). Moreover, since hyper-cross-linking could form micropores in the PS domain of molecular brushes, the higher weight fraction and longer PS blocks in FNSPs would contribute to increase surface areas. Thus, FNSP-PSL had a higher surface area than FNSP-PSS (115 vs 77 m2 g−1). The nanonetwork-structured polymer (NSP) prepared from P(BiBEM-g-PS220)1860 precursor, with only PS composition, had significantly higher surface area (SBET 274 m2 g−1) than both FNSPs. Water pollution, which can cause bodily damage and disease in humans and other species, has greatly threatened public health and the environment. Basic triphenylmethane dyes, a typical class of toxic organic pollutants, are primarily distributed into the environment through discharge from textile, plastics, leather, and printing industrial plants. Utilization of materials with a core−shell architecture and PAA functionality could enhance adsorption of positively charged molecules via Coulombic attractions. Methyl violet (MV), a positively charged triphenylmethane dye, was used to evaluate the adsorption properties of the functional nanonetworks toward basic organic pollutants. As shown in Figure 4a, under a constant concentration of MV (200 mg L−1), significantly faster adsorption was observed for FNSPs compared to NSP, although NSP had a larger surface area. After FNSP-PSL and FNSP-PSS were dispersed in MV solution for 5 min, both removed more than 60% of MV from the solution (504 vs 496 mg g−1), while NSP removed only 15%. After 24 h of adsorption (Figure 4b), NSP could only remove about half of MV from the solution, while FNSP-PSL and FNSP-PS S removed 96% and 78%, showing superior adsorption capacity of 767 and 625 mg g−1, respectively.

Remarkably, the MV adsorption capacity of FNSP-PSL was significantly higher than for other reported adsorbents (Table S5). The extraordinary adsorption capacity of PAA-containing networks could be attributed to both porosity and weight fraction of functional PAA domains. Interestingly, absorption kinetics followed a two-stage pattern in which fast adsorption early in the experiments was responsible for more than half of the final adsorption capacity. The incorporation of PAA domains substantially favored MV adsorption for FNSPs in this early stage fast adsorption, which confirms the existence of electrostatic interaction between carboxyl groups and positively charged active MV. Relatively slower adsorption (in the second stage) was responsible for about 11%, 27%, and 39% of the final adsorption capacity in FNSP-PSS, FNSP-PSL, and NSP, respectively. This trend suggests that increasing PS weight fractions and surface areas in the nanonetworks are still beneficial for physical dye absorption during the slow adsorption stage.

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CONCLUSIONS In summary, we have developed a novel class of FNSPs by combining ATRP and Friedel−Crafts reaction to hyper-crosslink well-defined molecular brush building blocks into nanonetworks composed of functional core-microporous shell network units. The FNSPs exhibited 3D interconnected hierarchical porous structures with micropores derived from intrabrush hyper-cross-linking of PS blocks, and meso-/ macropores induced by interbrush hyper-cross-linking. Dye adsorption studies indicated that incorporation of PAA functionality into the nanonetworks significantly enhanced the adsorption of positively charged MV dye via Coulombic interactions. It is envisioned that further tuning of the nanostructures and functionalities could be accomplished by variation of building blocks, both backbone and side chain composition. This novel strategy of preparing functional porous materials could provide new opportunities for highperformance nanonetworks in applications including separation, catalysis, and medicine.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03845. Experimental procedures used to prepare and characterize all materials; synthetic scheme, 1H NMR spectra, D


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GPC traces, and SEM image of P[BiBEM-g-(PtBA50-bPS172)]1640; TGA curves and FTIR spectra of P[BiBEMg-(PtBA50-b-PS172)]1640 and FNSP-PSL; Friedel−Crafts hyper-cross-linking mechanism; TEM image of FNSPPSS. Adsorption capacity of FNSP-PSL compared to other adsorbents (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.M.). *E-mail: [email protected] (D.W.). ORCID

Dingcai Wu: 0000-0003-1396-0097 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the project of the National Natural Science Foundation of China (U1601206, 51872336 and 51422307), the Leading Scientific, Technical and Innovation Talents of Guangdong Special Support Program (2017TX04C248), the National Program for Support of Top-notch Young Professionals, the Fundamental Research Funds for the Central Universities (18lgzd10), the National Key Basic Research Program of China (2014CB932400), and the National Science Foundation (DMR 1501324).

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